The Collaborative International Dictionary of English v.0.48:

Amphisbaena \Am`phis*b[ae]"na\, n. [L., fr. Gr. ?; ? on both
   ends + ? to go.]
   1. A fabled serpent with a head at each end, moving either
      way. --Milton.
      [1913 Webster]

   2. (Zool.) A genus of harmless lizards, serpentlike in form,
      without legs, and with both ends so much alike that they
      appear to have a head at each, and ability to move either
      way. See Illustration in Appendix.
      [1913 Webster]

   Note: The Gordius aquaticus, or hairworm, has been called
         an amphisbaena; but it belongs among the worms.
         [1913 Webster]

WordNet (r) 3.0 (2006):

amphisbaena
    n 1: (classical mythology) a serpent with a head at each end of
         its body
    2: type genus of the Amphisbaenidae [syn: Amphisbaena, genus
       Amphisbaena, Amphisbaenia, genus Amphisbaenia]